This invention relates to an improvement in nuclear reactor operations and, more particularly, to a shield for preventing or minimizing radiation emanating from the reactor itself or fission and/or activation products resulting from reactor operation.
One of the primary uses of the shield of this invention is to attenuate the flux of neutrons from openings communicating with the reactor vessel.
Normally, a nuclear reactor assembly includes a fuel containing reactor vessel which is surrounded by a protective housing which is commonly referred to as the primary shield. The primary shield forms a right cylinder around the reactor vessel external surface and is spaced from the vessel to define a reactor cavity.
The annular space between the operating reactor vessel and the primary shield must remain open so as to relieve pressures which may result from an accidental loss of coolant. Additionally, federally required inspection necessitates open access to critical areas. Normally this results in a path of free communication between the reactor and the "operating" floor, an area occasionally occupied by technical personnel.
Duration of occupancy of the containment may be severely and unacceptably limited by the level of radiation since strict health physics rules limit the radiation dosage to be received by any individual. This limitation of stay in the containment, in turn, reduces reactor operation efficiency.
The prior art is extremely primitive. In only several operating plants, to our knowledge, has the extent of the neutron flux been recognized. Others have attempted to alleviate the neutron radiation problem by utilizing rigid hydrogenous masses in the form of plates or blocks, sometimes with a heavy metal support structure. There are several problems with those types of shields.
First of all, they are extremely heavy and difficult to install and remove, resulting in significant time expenditures. Further, such shields create storage problems when not in use. Additionally, and perhaps most importantly, those prior art shields may become missiles in the event of a pressure blowout in the reactor cavity, endangering safety-related systems within the containment.
At least one prior art patent, U.S. Pat. No. 3,812,008 to Fryer discloses the use of an inflatable seal which is described as being useful during refueling operations when the upper portion of the reactor cavity is filled with water. The seal prevents water from passing through to the lower portion of the reactor cavity. Therefore, the inflatable seal of the Fryer patent is not disclosed as being used for a radiation shield. Further, the Fryer seal as well as the aforementioned prior art shields must be more precisely manufactured to conform with the dimensions of particular openings in the reactor cavity.
While this invention is useful as a neutron shield as described above, it is also useful against other types of radiation, including beta-gamma radiation, in all areas of the nuclear reactor installation. The fluid coolant of a nuclear reactor contains radioactivity which has leaked from the fuel assemblies or results from the activation of corrosion products. This reactor coolant requires constant purging to maintain required purity. The separation, treatment and disposal of radioactive waste products entail a number of systems generally labeled "auxiliary" or "radwaste" systems. Additionally, liquid or gaseous leaks from these fluid streams, both primary coolant and purification and radwaste must be collected, treated and disposed of as part of the radwaste system.
These radioactive systems in the nuclear reactor installation frequently require maintenance inspection or repair which necessitates personnel entry into areas of high radiation. The more concentrated sources of this radiation may be accumulated fission and corrosion products in a filter or resin bed or crud deposits in equipment, piping, or tankage. Shielding these sources is required to minimize radiation dosage to personnel. Quick and effective means of establishing emergency portable shielding permits more efficient and economic operation of the reactor installation.
Prior art devices include lead sheet and concrete block type shields which do not lend themselves readily to portability due to their weight. The container of this invention can be readily put in place and then filled with an attenuation fluid to form the shield of the invention, while the placement of heavy blocks or metal sheets is very time consuming and generally requires the use of mechanical handling and lifting apparatus. The time required to properly emplace such heavy prior art devices greatly increases personnel exposure to radiation. Since there are limitations on the amount of radiation exposure permitted to personnel over certain periods of time, the use of prior art devices may result in the radiation doses which may preclude personnel from working in the area for extended periods of time resulting in economic losses to the operator.